An isotactic polypropylene (hereinafter also referred to as i-PP) conventionally used as an insulating material is inherently rigid and subject to limitation when used as a bulky material for electrical insulation. It is often inadequate, moreover, in terms of rigidness and brittleness (susceptibility to occurrence of cracks), when formed into a thin sheet or a composite with other materials.
A low density polyethylene (LDPE) and ethylene propylene rubber (EP rubber) may be also used as an insulating material for general use. Yet, there remains a demand for an insulating material having higher electrical properties and physical properties, and formed articles including insulators formed therefrom.
The conventional insulating layer and coating layer for cables are associated with various problems to be noted below.
A low density polyethylene conventionally used as an insulating composition for power cables has a low melting temperature, which can possibly result in deformation of an insulator when the cable is energized and the conductor is heated thereby. To prevent this, polyethylene is mostly crosslinked (e.g. chemical crosslinking). The biggest problem in producing a power cable which requires such crosslinking step is the longer production time necessary for crosslinking reaction.
One of the polymers capable of overcoming this problem is polypropylene which is the same polyolefin as polyethylene. However, the i-PP currently on the market has poor flexibility and low electric breakdown property to the extent that frequent use thereof as a cable insulating composition is rare.
Accordingly, a polymer having a sufficiently high melting temperature, superior electric breakdown property even without crosslinking, and flexibility required of a cable insulating material is needed as an insulating material to solve the above-mentioned problems. In particular, the polymer is required to have superior electric breakdown property at high temperatures, since the cable insulator reaches about 70.degree. C. due to the heating of the conductor during operation.
As an insulating material for low voltage electric wires such as wires for equipments, specifically for underwater motors, a blend of isotactic polypropylene/ethylene propylene rubber (hereinafter also referred to as i-PP/EP rubber) has been conventionally used.
A low voltage electric wire comprising an insulating material of the above-mentioned i-PP/EP rubber is poor in resistance to underwater breakdown and cut-through property and is problematic. That is, a low voltage electric wire comprising an i-PP/EP rubber easily develops insulation breakdown when used in water for an extended period of time, and is vulnerable to external damages.
Conventionally used as an insulating layer of power cables is LDPE. When it is used for high voltage cables of 6.6 kV or above, for example, the temperature of the cable elevates due to heating caused by the electric resistance of the conductor, which can possibly cause softening of an LDPE insulating layer. For this reason, a crosslinked LDPE (XLPE) imparted with enhanced heat resistance by crosslinking LDPE is generally used as an insulating layer of high voltage power cables. A chemical crosslinking using an organic peroxide is most frequently employed for this end.
On the other hand, a high voltage power cable requires inner semiconductor layer and an outer semiconductor layer formed on the both sides of an insulating layer to relax the electric field. These inner and outer semiconductor layers are, like the aforementioned LDPE, generally crosslinked for improving heat resistance, and the crosslinking is performed simultaneously with the crosslinking of insulating layer.
The crosslinking requires high temperature and a long reaction time, thus markedly restricting the production efficiency of high voltage power cables.
Communication cables generally comprise a foamed insulating layer of, for example, a high density polyethylene (hereinafter referred to as HDPE) to cover and protect a conductor for the purpose of reducing noises.
When a foamed layer is formed from HDPE, the layer often has a less void ratio (hereinafter referred to as expansion ratio) in HDPE than desired and fails to achieve uniform foaming, which in turn results in insufficient action of the foamed layer to reduce occurrence of noises.
In addition, a low expansion ratio increases dielectric constant, which leads to the absorption of electric energy by the foamed layer, causing attenuation of communication signals and low transmission efficiency.
While a foam of HDPE having an expansion ratio of 20-30% is rather easy to manufacture, in general terms, a highly foamed article having an expansion ratio of 50% or more is difficult to manufacture. Consequently, the foam may have poor mechanical properties such as flexibility and cannot serve well as an insulating layer of communication cables.
As flame-resistant cables, there have been conventionally known those having a flame-resistant layer composed of a flame-resistant composition comprising a high polarity flame retardant such as decabromophenyl ether or magnesium hydroxide [Mg(OH).sub.2 ] and a base polymer such as HDPE. Such flame-resistant cable is defective in that mechanical properties drastically fall due to the above-mentioned flame retardant which needs to be added in a large amount to impart high flame resistance to the flame-resistant layer.
Optical fibers are vulnerable to damages, cracks, hitting and the like due to the external mechanical force such as bending and deformation during preparation, transport, installation and storage. So as to protect the optical fibers from such external mechanical force, for example, a resin jacket is formed directly on a cladding layer or via a primary layer. Known as such primary layer are, for example, ultraviolet (UV) curing epoxyacrylate resin and thermosetting silicone resin, and known as a jacket (secondary coating) is polyamide. The above-mentioned primary layer tends to develop pin-holes during curing. Presence of pin-holes in the primary layer results in inconsistent clamping of optical fiber, which in turn causes distortion of the optical fiber and increase in transmission loss. In addition, the above-mentioned jacket shrinks when cooled to a low temperature and bends optical fiber to cause increase in transmission loss. Moreover, the jacket is rigid, posing problems in terms of physical properties against stretching and bending.
The material for protecting electric wires, such as for sheath and jacket, is, for example, polyvinyl chloride (PVC) and HDPE. However, PVC requires relatively great amounts of plasticizer to improve processability and to impart flexibility. The plasticizer bleeds out when in use and adheres to or permeates into a semiconductor in an insulated portion to possibly affect the insulating property by increasing resistance. In addition, HDPE has poor environmental stress cracking resistance property (ESC resistance) due to the high crystallinity it has.
The polymer forming the insulating portions of a structure connecting electric wires is LDPE. Therefore, EMJ (extrusion mold joint) and BMJ (block mold joint) necessitate crosslinking of polyethylene to prevent thermal deformation of the insulator during power transmission, which crosslinking lengthens the production time. With regard to TMJ (tape mold joint) in the event a non-crosslinked material is used, it is associated with problems of cold flow and residual gap attributable to tape winding; and when a crosslinkable material is used, it is associated with a problem in that production time becomes longer. Moreover, PJ (prefabrication joint) is associated with a problem that the epoxy resin, a material constituting an insulating tube, weighs much.
Thus, a wire-connecting structure capable of overcoming the above-mentioned problems has been demanded.